#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pair<int,int>> vpi;
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)
#define pb push_back
#define rsz resize
#define sz(x) int(x.size())
#define all(x) begin(x), end(x)
#define fi first
#define se second
struct triple
{
int a,b,c;
};
bool operator<(const triple &x, const triple &y)
{
return x.a<y.a;
}
int const N=3e4;
int const sN=130;
int d[N][sN];
int const INF=1e9;
vector<triple>adj[N][sN];
bitset<sN> vis[N];
priority_queue<triple> q;
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
int n,m;
cin>>n>>m;
int b0,b1;
F0R(i,m)
{
int bi,pi;
cin>>bi>>pi;
if(i==0)b0=bi;
else if(i==1)b1=bi;
if(pi>=sN)
{
int nxt=bi+pi;
int cnt=1;
while(nxt<n)
{
adj[bi][0].pb({nxt,0,cnt});
++cnt;
nxt+=pi;
}
nxt=bi-pi;
cnt=1;
while(nxt>=0)
{
adj[bi][0].pb({nxt,0,cnt});
++cnt;
nxt-=pi;
}
}
else
{
adj[bi][0].pb({bi,pi,0});
}
}
F0R(i,n)
{
fill(d[i],d[i]+sN,INF);
}
d[b0][0]=0;
q.push({0,b0,0});
while(!q.empty())
{
int pos = q.top().b;
int pw = q.top().c;
q.pop();
if(vis[pos][pw])continue;
vis[pos][pw]=1;
if(pw!=0)
{
int nxt=pos+pw;
if(nxt<n)
{
adj[pos][pw].pb({nxt,pw,1});
adj[pos][pw].pb({nxt,0,1});
}
nxt=pos-pw;
if(nxt>=0)
{
adj[pos][pw].pb({nxt,pw,1});
adj[pos][pw].pb({nxt,0,1});
}
}
trav(e,adj[pos][pw])
{
int cand = d[pos][pw]+e.c;
if(cand<d[e.a][e.b])
{
d[e.a][e.b]=cand;
q.push({-cand,e.a,e.b});
}
}
adj[pos][pw].clear();
}
if(d[b1][0]==INF)cout<<-1<<'\n';
else cout<<d[b1][0]<<'\n';
return 0;
}
